If the jury shall be satisfied that the defendant, or those under whom he claims, made on the premises, at a time when there was reason to believe the title good under which he or they were holding the same, permanent and valuable improvements, they shall determine the value of such improvements as were so made before receipt by the person making the same of notice in writing of the title under which the plaintiff claims, not exceeding the amount actually expended in making them, and not exceeding the amount to which the value of the premises is actually increased thereby at the time of such determination.